This area of open draw has always been a source of problems in paper machine operation. The wet web of fibers is very weak at this point, and any imperfection or weakness in the wet web may cause the wet web to break. There has been a great deal of research and development done to overcome this problem and probably the most outstanding development is the pickup press. The pickup press actually comes into contact with the fourdrinier wire at the couch roll and by a slight vacuum attaches the wet web to the press felt so that it does not have to jump across to the press. The improvement offered by the pickup press is, however, not without cost as large and expensive equipment is required. As a result many of the older fourdrinier paper machines continue to operate as previously, with an open draw. Either they do not have the space for this additional equipment or it is considered too expensive.
It has also been common practice to locate a draw roll in between the couch and the first press, the purpose of which is to hold the wet web of paper down to maintain the proper angle of takeoff of the wet web at the couch roll as it leaves the fourdrinier wire.
However, in so accomplishing this necessary task the draw roll has given birth to other problems, many of which are serious in their own right. Some of these problems will be delineated in the following paragraphs.
Every time the wet web breaks, the draw roll has to be raised in order to pass the lead strip thereunder and then it must be repositioned. This physical act of bringing the draw roll back into position against the wet web can be a cause of trouble as the sheet is weak and delicate at this point and can easily break. It takes an experienced operator to pass the lead strip and make the necessary adjustments before normal paper manufacture can be resumed.
It is common for draw rolls to pick up deposits from the top of the wet web. These deposits are only lightly attached to the draw roll and frequently break off and cause holes, lumps and other imperfections in the wet web travelling underneath. Cleaning the draw roll while the wet web is running is difficult and may in fact be impossible. Furthermore the surface damage to the sheet cannot be repaired and it will thus be of inferior quality.
The diameter of the prior art draw roll must necessarily be small because of the very limited space available. This causes a deflection problem, with the draw roll sagging in the middle and depressing the wet web more in the center. Depending upon the grade of paper being run and the equipment available this gives rise to a whole series of other potential problems, such as paper which is baggy in the center, flapping and flutter in the dryers, uneven tension, wrinkles, edge cracks, pull apart, sheet drooping on the sides, air entrainment, etc. These are some of the disadvantages caused by draw rolls.